The present invention relates to resonant cavities for microwaves which are temperature stabilized, do not require hermetic sealing, are easy to be frequency adjusted and comprise a hollow body, a tuning screw, a plug, auxiliary lateral devices for coupling to the diode and a termination. It is known that at present many types of microwave cavities are implemented. Among those with a metallic wall the most important ones are:
(1) TEM mode coaxial cavity;
(2) TE.sub.10 mode waveguide cavity;
(3) TE.sub.11 mode circular waveguide cavity;
(4) TE.sub.01 mode circular waveguide cavity.
These cavities are implemented to form microwave circuits, viz.:
in stable oscillators they appropriately couple the cavities to the active circuit that generates oscillation so that the oscillator frequency is determined almost only by the cavity.
In the filters they appropriately couple a suitable number of cavities one to the other; particularly they couple the first cavity to the generator and the last one to the load. The biggest problem to be solved with such structures is the cavity resonance frequency stabilization upon a variation of environmental conditions (temperature and humidity) whenever a high frequency stability in the 1 ppm/.degree.C. order is to be attained.
In fact, there generally are three fundamental factors affecting the resonance frequency of a cavity, viz.:
(1) Expansion due to temperature of the cavity's metal;
(2) Dielectric constant of the gas filling the cavity;
(3) Load impedance at the ports coupling the cavity to the outside.
As far as item (3) is concerned, the load affect becomes negligible by adequately reducing the coupling amount towards the load and, where necessary, by introducing an isolator between cavity and load.
As to item (1) it has already been suggested to manufacture the cavity body in a metal with a low expansion coefficient vs temperature e.g. in an iron-nickel alloy known under the commercial names of Invar and Super Invar with an expansion coefficient less than/equal to 1.5 ppm/.degree.C. and less than/equal to 0.7 ppm/.degree.C., respectively.
In addition a particular heat treatment for stabilization of these materials are previewed before and after their being worked. In this way also the end product maintains the expansion coefficient values specified.
Finally concerning item (2) it is necessary to hermetically seal the cavity (i.e. it must be moist- and gasproof) before filling it with a dry inert gas (e.g. nitrogen) thus cancelling the difference in pressure with respect to the external environment.
This solution is particularly hazardous as all soldering of the several parts constituting the cavity as well as the coupling irises and tuning adjustments must be sealed. At first object of this invention is to provide a cavity that does not present the mentioned inconveniences, whilst it is temperature stabilized with very simple and efficient means. A second object of the invention is to provide a cavity which not only is efficiently stabilized in temperature, but can also be easily frequency adjusted.